By now you have probably heard the warning: Playing Mirror's Edge will make you vomit.

The hot new videogame is a sort of "first-person runner": You're a courier who travels across the rooftops of a locked-down, police-state city, delivering black-market messages by using acrobatic feats of parkour. You're constantly leaping over gaps 40 stories in the air, tightrope-walking along suspended pipes and vaulting up walls like a ninja.

It doesn't do justice to call the action in Mirror's Edge "intense": It quivers, like a hummingbird, and your first-person view is constantly whipsawing like a paranoid cameraman hunting for the best shot.

Only 15 minutes into the game, my mouth began overproducing saliva, and I had to pause the action for a few seconds to avoid carsickness. I would feel like a total lamer, but apparently even the Penny Arcade guys wrestled with nausea.

Still, it made me wonder: What makes Mirror's Edge so different? Sure, the action is swoopy and vertiginous, just as it is in many other games. But I've played plenty of first-person shooters that required me to navigate ridiculous, zero-G boss lairs that were suspended over improbable heights, and none of those ever made me feel nauseated.

Why does this game get its hooks into my brain so effectively? Why does it feel so much more visceral?

I think it's because Mirror's Edge is the first game to hack your proprioception.

That's a fancy word for your body's sense of its own physicality — its "map" of itself. Proprioception is how you know where your various body parts are — and what they're doing — even when you're not looking at them. It's why you can pass a baseball from one hand to another behind your back; it's how you can climb stairs without looking down at your feet.

Most first-person shooters do not create any sense of proprioception. You may be looking out the eyes of your character, but you don't have a good sense of the dimensions of the rest of your virtual body — the size and stride of your legs, the radius of your arms. At most, you can see your arms carrying your rifle out in front of you. But otherwise, the designers treat your body as if it were just a big, refrigerator-size box.

Worse, in most games your virtual body cannot do even the most simple things that it ought to be able to do. Every time I'm playing a first-person shooter, I'll inevitably try to jump or walk up onto an object — a ledge, a curb, a railing along a wall — and discover that I can't. The designers decided they didn't need to worry about those subtle physics, and the resulting limitation completely breaks the illusion that I'm in that virtual body.

Mirror's Edge, in contrast, does something very subtle, but very radical. It lets you see other parts of your body in motion.

When you run, you see your hands pumping up and down in front of you. When you jump, your feet briefly jut up into eyeshot — precisely as they do when you're vaulting over a hurdle in real life. And when you tuck down into a somersault, you're looking at your thighs as the world spins around you.

What's more, the Mirror's Edge world feels tactile and graspable. Because the game is designed around the concept of parkour, or moving through obstacles, most times when you see something that looks like you could jump on it, you can. The gameplay requires it.

The upshot is that these small, subtle visual cues have one big and potent side effect: They trigger your sense of proprioception. It's why you feel so much more "inside" the avatar here than in any other first-person game. And it explains, I think, why Mirror's Edge is so curiously likely to produce motion sickness. The game is not merely graphically realistic; it's neurologically realistic.

Indeed, the sense of physicality is so vivid that, for me anyway, the most exhilarating part of the game wasn't the obvious stuff, like leaping from rooftop to rooftop. No, I mostly got a blast from the mere act of running around. I've never played a game that conveyed so beautifully the athletically kinetic joys of sprinting — of jetting down alleyways, racing along rooftops and taking corners like an Olympian. It's an interesting lesson of game physics: When you feel like you're truly inside your character, speed suddenly means something.

The opposite is also true. Without a sense of physicality, speed feels lifeless. In Halo, you're playing as the cyborgically enhanced Master Chief, so your top speed at an open run is — according to Halo nerd canon — 30 mph or something. But it doesn't feel very fast at all, because your avatar doesn't appear to be actually exerting himself. When you run, your body bobs along not much differently from how it moves when you're walking, except the scenery goes by more quickly.

The combat in Mirror's Edge felt more believable than doing battle in Halo, too. When the cops were shooting bullets at me and I was frantically racing to escape, I kept thinking: "Damn, I'm going so fast I might just escape!" In most first-person games, I usually wonder the opposite: How are these guys not hitting me? So the brilliant physicality of Mirror's Edge isn't just a boon to the game's physics. It also makes the narrative and drama more plausible.

So yes, by all means, I'll keep on playing Mirror's Edge, even though it occasionally makes me want to vomit. In the past, I've often wanted to wretch because a game is so bad — but I've never felt sick because it was so good.

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